Recent years have seen the acceleration of development of bulb-type fluorescent lamps and bulb-type LED (Light-Emitting Diode) lighting apparatuses which attain a high degree of energy savings in illumination. An organic EL (Electroluminescence) lighting apparatus which attains a similarly high degree of energy savings is a surface-emitting light source, and has advantageous properties of being thin, lightweight and flexible. With such properties, organic EL provides new illumination designs, showcases and showrooms conventionally unattainable, and allows new presentations at a store and the like. The development of organic EL has also been promoted since it is environmentally friendly in that it is mercury-free.
FIG. 6 is a schematic diagram showing the cross-sectional structure of a common organic EL element 100. As shown in FIG. 6, organic EL element 100 includes ITO 101, an organic EL layer 102 and an aluminum layer 103, which are stacked on a base member 104. Organic EL element 100 is configured such that a direct-current voltage is applied between aluminum layer 103 and ITO 101.
The light emission principle of an organic EL element is to pass a current through an extremely thin film having a transparent conductive film (anode), an organic EL material and a metal electrode (cathode) stacked on a substrate, to produce recoupling of electrons and holes in the organic EL material to thereby generate light.
FIG. 7 is a graph showing an example of current-voltage characteristics, which represent a relationship between voltage and current density of organic EL element 100 for each temperature. FIG. 8 is a graph showing an example of a relationship between current density and luminance of organic EL element 100 for each temperature.
As shown in FIG. 7, the current-voltage characteristics of organic EL element 100 are similar to the current-voltage characteristics of a diode. Namely, there is very little current flow at a low voltage, and then a current flows abruptly when a certain threshold voltage is reached. Furthermore, as can be seen from FIG. 7, when the current density is measured with the temperature varying from a room temperature of 26.0 degrees to 81.0 degrees, the threshold voltage is shifted downward as the temperature increases, and when the voltage is constant, the current increases abruptly as the temperature varies. As shown in FIG. 8, on the other hand, the current density and the luminance have an approximately linear relationship, and exhibit relatively stable behavior with respect to temperature.
If organic EL element 100 exhibiting such behavior is driven by voltage control, a voltage value needs to be controlled with a very high degree of accuracy, and compensation for temperature needs to be made with a similarly high degree of accuracy. Furthermore, if organic EL element 100 is driven by voltage control, electro-optical conversion efficiency is significantly affected by a variation in threshold voltage during manufacture of organic EL element 100. For this reason, organic EL element 100 should be driven by current control to obtain a more stable luminance with respect to temperature variation and the like.
FIG. 9 is a block diagram of a common current driving device for driving organic EL element 100. Organic EL element 100 to be driven, a power supply 202 and a current control unit 203 are connected in series. Current control unit 203 includes a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) 205, a current control circuit 204 and a current detection resistor 201.
Current control unit 203 measures a voltage Vs across current detection resistor 201 having a resistance value Rs, in order to pass a constant current Ioled through organic EL element 100 by using current control circuit 204. Current control unit 203 compares measured voltage Vs with a reference voltage Vref, and controls a gate voltage of MOSFET 205 in response to a signal from the current control circuit to satisfy voltage Vs=reference voltage Vref, to vary a current passing through MOSFET 205. When Vref=Vs is satisfied, current Ioled passing through organic EL element 100 is obtained by an equation below.Ioled=Vref/Rs
As can be seen from this equation, current Ioled can be varied by varying the value of Vref.
A voltage Vo supplied to organic EL element 100 from power supply 202 is the sum of a voltage drop Voled in organic EL element 100, a voltage drop Vmos in MOSFET 205, and a voltage drop Vs in current detection resistor 201. In reality, voltage Vo needs to be set to a sufficiently large value by taking into account a margin to adapt to the adjustment of luminance of organic EL element 100, a variation during a manufacturing stage, temperature characteristics, temporal variation and the like of organic EL element 100.
Power based on the voltage set to a sufficiently large value with a margin, however, becomes heat at MOSFET 205 and/or current detection resistor 201 and is consumed. That is, power based on this extra voltage is wasted without contributing to light emission.
In particular, when luminance adjustment for organic EL element 100, i.e., dimming needs to be performed, an approximately double-digit dimming range is required, which requires a double-digit range for current. During dimming, therefore, a voltage detected by current detection resistor 201 varies in double digits. Since power consumption is proportional to the square of voltage, double-digit variation in voltage causes four-digit variation in power, resulting in power wasted particularly in a low luminance region.
PTL 1 discloses a driving device including a group of a plurality of light-emitting elements (light-emitting diodes) connected to a plurality of constant current supplies, respectively, in which switching control of a power supply circuit is performed such that the lowest one of dropped voltages of the constant current supplies is constant. It is stated that, according to this technique of controlling the dropped voltage of the constant current supply to a constant value, a loss in voltage detection by detecting the resistance is not increased even if a current passing through the light-emitting diodes increases.
Japanese Laid-Open Patent Publication No. 2008-134288 (PTL 2) discloses an LED driver for supplying power to an LED, which includes a constant current circuit unit connected in series with the LED for adjusting a current passing itself to a prescribed value, and a voltage regulation unit connected in series with the constant current circuit unit for regulating the voltage by way of a switching regulator. It is stated that, according to this LED driver of regulating the voltage by the switching regulator, waste of power consumption involved with a voltage drop process can be minimized.